Conventional overvoltage protection devices typically use a gas tube, or gas discharge, surge arrestor as a primary means for diverting voltage surges from a signal line to ground. Examples of such devices are shown in U.S. Pat. Nos. 5,388,023, 5,500,782 and 5,880,919. Gas tubes dissipate energy by producing electrical arcing to ground. A gas of known dielectric strength is ionized when subjected to an electrical surge. One drawback of gas tubes, however, is that they typically exhibit a relatively slow response time and thus, may not be able to safely suppress fast rise time voltage surges. Metal oxide varistors (MOVs) have therefore been used as secondary protectors in back-up and interacting overvoltage protection devices. For example, in a conventional hybrid station protector, an MOV is electrically connected in parallel with the gas tube between each signal line. Although the gas tube can repeatedly dissipate voltage surges without damage, the response time of the MOV is faster than that of the gas tube. Therefore, the MOV can be relied upon to shunt fast rise time voltage surges to ground, while the parallel gas tube is relied upon to shunt sustained voltage surges, which might otherwise damage the MOV.
Overvoltage protection devices utilizing MOVs as secondary protectors have been successfully employed to protect conventional twisted-pair (i.e., “tip” and “ring”) telecommunications lines. Broadband communications operate at transmission frequencies of at least 1 megahertz, which is substantially higher than the frequencies traditionally employed over twisted-pair telephone lines. Presently, frequencies of about 30 megahertz are typically utilized for xDSL communications transmitted over twisted-pair telephone lines. Existing twisted-pair telephone lines, also referred to as outside plant wire, are typically CAT-3 grade or less and were not intended for high frequency performance when originally manufactured or installed. Although xDSL communications are possible over existing twisted-pair telephone lines, in many instances conventional overvoltage protection devices are inadequate. This is especially the case when existing twisted-pair telephone signal lines are used for higher frequency digital transmissions, such as VDSL. Even if only a small number of overvoltage protection devices perform inadequately, the cost of identifying and replacing the overvoltage protection devices that may be adequate for lower frequency xDSL transmissions, but inadequate for higher frequency xDSL transmissions, is significant.
The inadequate performance of some conventional overvoltage protection devices, such as station protectors utilized at customer premises for higher frequency xDSL communications, has been traced to the greater capacitance and the variability of the capacitance of the MOVs that are employed in the station protector. At higher frequencies, the greater capacitance and the variability of the capacitance results in unacceptable insertion loss, return loss, and longitudinal imbalance. It is well known that the capacitance can be reduced by utilizing MOVs having the same thickness, but a smaller diameter. Many conventional station protectors employ 5 mm diameter MOVs with symmetrical 3.8 mm electrodes instead of smaller MOVs to absorb additional energy without causing permanent damage. MOVs of this size have a capacitance of about 60 picofarads with a tolerance of about 20 percent. This relatively large tolerance is believed to be due to variability in the varistor material and thickness, and/or to the relative placement and size of the electrodes on opposite sides of the varistor material. Electrodes, which are intended to be identical on both sides of the varistor material, can in practice be laterally displaced relative to each other. The concentricity of the two electrodes can also vary. Uneven placement and varying concentricity of the electrodes on opposite sides of the varistor material means that the overlapping surface area of the electrodes can vary significantly between MOVs that are intended to be identical, thereby generating dissimilar electric fields that result in relatively high capacitive tolerance, variability or spread. The difference in the capacitance of the MOV between the tip conductor and ground and between the ring conductor and ground results in significant capacitance mismatch, referred to herein as capacitive imbalance, which can cause excessive signal loss (e.g., insertion loss and return loss) and longitudinal imbalance at the higher frequencies utilized for xDSL communications transmitted over twisted-pair telephone lines.
As previously mentioned, it would be possible to reduce the capacitance between a signal line and ground in a station protector if an MOV having a smaller diameter was employed. Given the same thickness, because the smaller diameter MOV inherently has electrodes with smaller overlapping surface areas, the smaller diameter MOV also has less capacitance. However, a smaller diameter MOV is not able to withstand the same sustained current as a larger 5 mm diameter MOV. Furthermore, substitution of the smaller diameter MOV would result in significant engineering, re-tooling and testing expense. Even if the desired reduction in capacitance could be achieved by substituting a smaller diameter MOV for the 5 mm MOV presently in use, there could still be an excessive capacitive imbalance between the tip conductor and ground and the ring conductor and ground. Accordingly, it would be preferable if both a reduction in capacitance and a reduction in the capacitive imbalance could be achieved without the need for extensive modifications to conventional station protectors.
It has been determined that station protectors will perform satisfactorily for higher frequency xDSL communications over twisted-pair telephone lines if the capacitance across the MOV in parallel with the gas tube is reduced to about 30 picofarads with a capacitive tolerance among the MOVs of about ±0.25 picofarads. The number of existing station protectors and other overvoltage protection devices which are incapable of adequate performance is excessive, at least in the aggregate. In particular, when MOVs having relatively large capacitance and large capacitive tolerance are employed in twisted-pair telephone lines, an unacceptable capacitive imbalance between the tip conductor and ground and the ring conductor and ground will be present in an excessive number of station protectors. The capacitive imbalance for such station protectors has been found to be up to about 5 picofarads. For xDSL communications, including VDSL, a capacitive imbalance of less than about 1.3 picofarads is desired. Accordingly, what is needed is an MOV that reduces the capacitance and capacitive imbalance in an overvoltage protection device while sustaining the same current without permanent damage to the MOV. Such an MOV would provide adequate performance in a station protector or other overvoltage protection device utilized on twisted-pair telephone lines that transmit higher frequency xDSL communications, such as VDSL.